Twist drill

ABSTRACT

A twist drill includes a cylindrical body having an axis of rotation therethrough and having a forward end which contacts a workpiece. The body has a spiral flute formed in an outer peripheral surface thereof so as to extend spirally along a length thereof to the forward end and a land disposed adjacent to the flute. The flute had a first wall facing in the direction of rotation of the body and a second wall extending from an inner end of the first wall to the outer periphery of the body. The first wall terminates at the forward end in a first cutting lip having a radially outermost end disposed on the outer periphery of the body. The second wall is concavely shaped when viewed from the forward end and formed so that, assuming a first line extending from the outermost end perpendicular to a second line which connects the outermost end and the axis of the body, the maximum distance between the first line and the second wall is set to range between 0.45D and 0.65D, wherein D is a diameter of the body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains generally to twist drills made of highspeed steel (HSS), sintered metal HSS, cemented carbide or cermet, andin particular to improvements to reduce cutting resistance exertedthereon during drilling operation.

2. Prior Art

Twist drills made of HSS or sintered metal HSS have hitherto beendeveloped as drills for effecting heavy-duty drilling operations. FIGS.1 and 2 depict a conventional twist drill of such type which has acylindrical body 1, a pair of spiral grooves or flutes 2 formed in theouter peripheral surface of the body 1 and a pair of lands each disposedbetween the pair of flutes 2. That wall portion of each flute 2 facingin the direction of rotation of the body 1 terminates at the forward endin a cutting edge or lip 3. Each spiral flute 2 is so formed that itswall is concavely shaped. In the drill for heavy-duty operations, theweb thickness T of the drill body 1 is made greater than a HSS drill fornormal drilling operations so that it is about 30% of the drilldiameter, while the flute-width ratio at the foward end defined by thearc length A of the flute to the arc length B of the land is set to beabout 0.7. Furthermore, at a cross-section of the drill, away from theforward end, the ratio of the arc length A₁ of the flute to the arclength B₁ of the land is set to about 1.16. With this construction, thetorsional rigidity of the body 1 of the twist drill is considerablyenhanced.

Furthermore, twist drills formed of cemented carbide or cermet have alsobeen extensively employed for heavy-duty drilling operations. Suchdrills are superior in wear resistance to HSS drills, but, due to theinferior mechanical strength, e.g., transverse rupture strength, agreater web thickness and a smaller flute-width ratio are necessary.FIGS. 3 and 4 illustrate a prior art drill of such type as disclosed inExamined Japanese Patent Application Publication No. 61-30845, in whichthe symbols in common with those in FIGS. 1 and 2 denote the same orlike parts. In this drill, the web thickness T and the flute-width ratioA/B are set to range between 20 and 35% of the drill diameter andbetween 0.4 and 0.8, respectively, while the flute-width ratio A₁ /B₁ ata cross-section away from the forward end is approximately 0.6.

In the twist drills of the afore-described types, however, there isalways the problem that the drill body 1 is susceptible to breakage whensubjected to heavy-duty drilling operations.

More specifically, chips or cuttings produced by the cutting lips 3during a drilling operation are produced as if a sector-shaped foldingfan were opened since their outer sides grow faster than the innersides, and curl at their tip ends by the bottom 2a of the flute 2, i.e.,by that portion of the flute wall where the distance between a line Lperpendicular to a radial line N, connecting the axis O of the body 1 toa radially outermost end Q of the cutting lip 3, and the flute wall isgreatest, so that the chips are broken at their roots by the resistancecaused due to the curling. The chips thus formed are illustrated in FIG.5, and are classified as "transition curled fractured type chips". Inthe above twist drills, the distance W between the line L and the bottom2a of the flute 2 is made rather small in order to enhance torsionalrigidity. As a result, the force exerted on the chips by the bottom 2aof the flute 2 acts in the direction opposite to the direction in whichthey grow, and hence thick chips, compressed strongly in thelongitudinal direction, are produced. The addition of the relativelylarge force on the chips causes the drill body 1 to be subjected to agreat cutting torque and thrust load.

Furthermore, in the above twist drills, the cross-sectional area of theflute 2 away from the forward end, which serves to discharge the chips,is rendered inevitably smaller, so that the chips may be jammed therein.This often causes the drill to break when the heavy-duty drillingoperation is effected.

SUMMARY OF THE INVENTION

It is therefore a primary object of the invention to provide an improvedtwist drill which is less susceptible to breakage during heavy-dutydrilling operations.

Another object of the invention is to provide a twist drill which canreduce the cutting resistance substantially by curling chips easily, andensuring a smooth discharge of the chips through flutes during thedrilling operation.

According to the present invention, there is provided a twist drillcomprising a cylindrical body having an axis of rotation therethroughand having a forward end which contacts a workpiece, the body having aspiral flute formed in an outer peripheral surface thereof so as toextend spirally along a length thereof to the forward end and a landdisposed adjacent to the flute, the flute having a first wall facing inthe direction of rotation of the body and a second wall extending froman inner end of the first wall to the outer periphery of the body, thefirst wall terminating at the forward end in a first cutting lip havinga radially outermost end disposed on the outer periphery of the body,the second wall being concavely shaped when viewed from the forward endand formed so that, assuming a first line extending from the outermostend perpendicular to a second line which connects the outermost end andthe axis of the body, the maximum distance between the first line andthe second wall is set to range between 0.45 D and 0.65 D wherein D is adiameter of the body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a forward end view of a conventional twist drill;

FIG. 2 is a cross-sectional view of the drill of FIG. 1;

FIG. 3 is a view similar to FIG. 1, but showing a different conventionaltwist drill;

FIG. 4 is a cross-sectional view of the drill of FIG. 3;

FIG. 5 is a view showing a chip produced during a drilling operation;

FIG. 6 is a forward end view of a twist drill provided in accordancewith a first embodiment of the present invention;

FIG. 7 is a side elevation of the drill of FIG. 6 as seen from thedirection indicated by the arrow VII in FIG. 6;

FIG. 8 is a cross-sectional view of the drill of FIG. 6 taken along theline VIII--VIII in FIG. 7;

FIG. 9 is a forward end view of a modified twist drill in accordancewith a second embodiment of the present invention;

FIG. 10 is an enlarged view of the circled portion labeled as X in FIG.9;

FIG. 11 is a forward end view of another modified twist drill inaccordance with a third embodiment of the present invention;

FIG. 12 is a view of a part of the drill of FIG. 11 as seen from thedirection designated by XII in FIG. 11;

FIG. 13 is a side elevation of the drill of FIG. 11;

FIG. 14 is a cross-sectional view of a part of the drill of FIG. 11taken along the line XIV--XIV in FIG. 11;

FIG. 15 is a view of the drill of FIG. 11 as seen from the directionindicated by the arrow XV in FIG. 12;

FIG. 16 is a view of the drill of FIG. 11 as seen from the directionindicated by the arrow XVI in FIG. 12;

FIG. 17 is an enlarged view of a chisel portion of the drill of FIG. 12;

FIG. 18 is a view similar to FIG. 17, but showing a differentarrangement of the chisel;

FIG. 19 is a forward end view of a further modified drill in accordancewith a fourth embodiment of the present invention;

FIG. 20 is a side elevation of the drill of FIG. 19 as seen from thedirection designated by XX in FIG. 19;

FIG. 21 is a cross-sectional view of the drill of FIG. 19 taken alongthe line XXI--XXI in FIG. 20;

FIG. 22 is a forward end view of a further modified twist drill inaccordance with a fifth embodiment of the present invention;

FIG. 23 is a side elevation of the drill of FIG. 22 as seen from thedirection designated by XXIII in FIG. 22;

FIG. 24 is a view similar to FIG. 22, but showing a modification of thedrill of FIG. 22;

FIG. 25 is also a view similar to FIG. 22, but showing a furthermodification of the drill of FIG. 22;

FIG. 26 is an enlarged view of the circled portion labeled XXVI in FIG.25;

FIG. 27 is a view as seen from the direction designated by XXVII in FIG.25, but showing a further modified drill in accordance with a sixthembodiment of the present invention;

FIG. 28 is a side elevation of the drill of FIG. 27;

FIG. 29 is a cross-sectional view of the drill of FIG. 27 taken alongthe line XXIX--XXIX in FIG. 25;

FIG. 30 is a view of a part of the drill of FIG. 27 as seen from thedirection designated by the arrow XXX in FIG. 27;

FIG. 31 is a view of a part of the drill of FIG. 27 as seen in thedirection designated by the arrow XXXI in FIG. 27;

FIG. 32 is an enlarged view showing a chisel portion of the drill ofFIG. 27;

FIGS. 33 and 34 are views similar to FIG. 32, but showing furthermodifications of the chisel portion, respectively;

FIG. 35 is a forward end view of a further modified twist drill inaccordance with a seventh embodiment of the present invention;

FIG. 36 is a side elevation of the drill of FIG. 35 as seen from thedirection designated by the arrow XXXVI in FIG. 35;

FIG. 37 is a cross-sectional view of the drill of FIG. 35 taken alongthe line XXXVII--XXXVII in FIG. 36; and

FIG. 38 is a view similar to FIG. 35, but showing a further modificationof the drill of FIG. 35.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In FIGS. 6 to 8, there is illustrated a twist drill in accordance with afirst embodiment of the present invention. The drill includes acylindrical body 10 of high speed steel (HSS) or sintered metal HSShaving an axis O of rotation therethrough and a forward end whichcontacts a workpiece. The body 10 has a pair of spiral flutes 11 formedin the outer peripheral surface of the body and extending spirally alongits length to the forward end, and a pair of lands 10a each disposedbetween the pair of flutes. Each flute 11 has a first wall facing in thedirection of rotation of the body 10 and extending from the outerperiphery of the body 10 generally radially inwardly and a second wallextending from an inner end of the first wall to the outer periphery ofthe body 10. Each of the first walls terminates at the forward end in afirst or primary cutting lip 12 which has a radially outermost end Qdisposed on the outer periphery of the body 10. Each second wall is asmooth continuation of the first wall and is concavely shaped whenviewed from the forward end. In order to provide a greater torsionalrigidity of the body, the body 10 has a web thickness T of 15 to 30% ofthe drill diameter D and a flute-width ratio A/B of 0.4 to 0.9. Thoseportions of relief surfaces or flanks disposed rearwardly with respectto the direction of rotation of the body 10 are ground off, as at 13, toprovide cross- or X-type ground surfaces 15, and hence second cuttinglips 13a, each extending away from the axis O of rotation to theradially innermost end of a respective first cutting lip 12, are formedat the web portion.

Furthermore, consider a line N connecting the axis O of the body 10 andthe radially outermost end Q of the cutting lip 12 and a line Lextending from the radially outermost end Q of the cutting lip 12perpendicular to the line N in an end view seen from the forward end ofthe body. Then, the distance W between the line L and the bottom 11a ofthe flute 11, i.e., the maximum distance between the line L and the wallof the flute 11, is made to range from 45 to 65% of the drill diameterD. With this arrangement, the spiral flute 11 is so shaped that thesecond wall, including the bottom 11a, is deeply recessed in thedirection of rotation of the body 10. Additionally, the entire surfaceof the drill body 10 is coated with a hard coating composed of at leastone material selected from the group consisting of TiC, TiN, TiCN andAl₂ O₃. Such a coating may be made of carbide, nitride or carbo-nitrideof other metal selected from Group IVa of the Periodic Table, and may belimited to the forward end of the body 10.

In the twist drill as described above, a chip produced by the cuttinglip 12 grows and is curled at the bottom 11a of the flute 11, so that itis broken into pieces of a transition curled fractured type asillustrated in FIG. 5. Inasmuch as the distance W between the line L andthe bottom 11a of the flute 11 is set to be no less than 45% of thedrill diameter, the freedom of the chip to curl inwards is greater thanfor conventional drills. As a result, the resistance exerted on the chipin the direction opposite to the direction in which the chip grows isdivided into forces to cause the chip to bend or to buckle. Accordingly,the load on the chip is not as great as for conventional drills so thatthe thrust load and cutting torque can be greatly reduced.

These points were verified by way of the following drilling tests.

DRILLING TEST 1

There were prepared several twist drills having various ratios of thedistance W to the drill diameter. The test drill had a diameter of 12 mmand a point angle of 140° , and the radial rake angle of the cutting lipwas -15°. The drilling tests were conducted under the followingconditions:

Cutting speed: 35 m/min.

Workpiece: Steel (JIS SCM440; Hardness: H_(B) 100 )

Feed rate: 0.15, 0.25, 0.35, 0.45 and 0.55 mm/revolution

The thicknesses of chips produced during the drilling operation usingthe above various drills were measured at a point designated by S inFIG. 5. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                Thickness of chips (mm)                                                          Drills of the invention                                                                       Prior art drills                                   Feed rate  Ratio (%)       Ratio (%)                                          (mm/rev.) 53       50     45      43   41                                     ______________________________________                                        0.15      0.261    0.293  0.309   0.423                                                                              0.445                                  0.25      0.355    0.369  0.387   0.528                                                                              0.542                                  0.35      0.427    0.440  0.466   0.634                                                                              0.654                                  0.45      0.524    0.525  0.553   0.756                                                                              0.789                                  0.55      0.650    0.658  0.698   0.955                                                                              0.997                                  ______________________________________                                    

As will be seen from Table 1, when the ratio, i.e. the distance W fromthe line L to the bottom 11a of the flute 11, is no less than 45% of thedrill diameter, the thicknesses of the chips are reduced substantially.This means that the resistance exerted on the chip can be substantiallyreduced by setting the distance W to be no less than 45% of the drilldiameter.

DRILLING TEST 2

The drill having a distance W of 53% of the drill diameter and the drillhaving a distance W of 41% of the drill diameter, both of which wereprepared for Drilling Test 1, were again used, and the drilling testswere conducted under the same conditions as those in Drilling Test 1. Inthis test, the thrust load, cutting torque, cutting power and maximumamplitude of vibration of the spindle during the drilling operation weremeasured. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                     Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      120      164    208  308    356                                 Prior art drill                                                                             216      280    352  416    480                                 Cutting torque (Kg · mm)                                             Drill of the invention                                                                       80      116    152  236    268                                 Prior art drill                                                                             116      184    218  264    354                                 Cutting power (Kw)                                                            Drill of the invention                                                                      0.45     0.68   0.86 1.22   1.39                                Prior art drill                                                                             0.74     1.19   1.37 1.62   2.07                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      3.0      2.64   2.64 2.76   2.34                                Prior art drill                                                                             3.6      5.76   2.7  4.2    6.54                                ______________________________________                                    

It is seen from Table 2 that in the drill of the invention, the thrustload, cutting torque and cutting power are all reduced markedly incomparison with the prior art drill. This is because, by the drill ofthis invention, chips can be easily curled without being deflectedsharply, and the force exerted on the chips can be reduced.

As verified in the above drilling tests, for the twist drill inaccordance with the present invention, the cutting resistance duringdrilling operations can be reduced significantly. In addition, since thedistance between the line L and the bottom 11a of the flute 11 is set tobe no greater than 65% of the drill diameter D, the thickness of thebody 10 between the second wall of the flute near a heel 11b and theperipheral land 10a is adequate to avoid chipping damage or cracking,and high torsional rigidity of the drill can be maintained.Additionally, since the surface of the drill body is coated with TiC,TiCN or the like, wear resistance is sufficiently enhanced, and thisenables heavy-duty operations to be performed.

Furthermore, as also seen from the results of the above drilling tests,the vibration of spindle of the machine tool is reduced. Accordingly,not only is the chipping damage of the cutting lip reduced, but alsodrilling precision is enhanced. Moreover, since the flute 11 is soformed that the wall portion including the bottom 11a is recessed deeplyin the direction of rotation of the body, the cross-sectional area ofthe flute 11 is large, and hence chips can be smoothly removed.

FIGS. 9 and 10 depict a twist drill in accordance with a secondembodiment of the invention. In this drill, the first cutting lip 12 andthe second cutting lip 13a, formed by the cross-thinning or X-thinningas at 13, are formed so that when viewed from the forward end of thebody 10, they are straight edges and the intersection 14 between thefirst cutting lip 12 and the second cutting lip 13a is curved at aprescribed radius of curvature r.

With this construction, the width of the chisel is rendered very smallby the cross thinning, and hence the thrust load can be further reduced.In addition, since the first cutting lip 12 and the second cutting lip13a are straight, the thickness of the chip becomes uniform in thetransverse direction of the chip. For this reason, the chip tends tobuckle easily when bent, so that it curls readily without being forced.Furthermore, since the intersection 14 of the first cutting lip 12 withthe second cutting lip 13a is curved, chips are not prone to separationat the intersection 14, thereby preventing the jamming of the chip andhence the breakage of the drill.

In this illustrated embodiment, the radius of curvature r of theintersection 14 is set to satisfy the relationship 0.05 D≦r≦0.15 D. Ifthe radius of curvature r exceeds 0.15 D, the effective portion of thecutting lip 12, which has a rake angle corresponding to the helix angleof the flute 11, is unduly reduced, and cutting resistance is increased.On the other hand, if the radius of curvature r is less than 0.05 D,chipping damage or separation of the chip is apt to occur at theintersection 14.

FIGS. 11 to 18 illustrate a twist drill in accordance with a thirdembodiment of the invention. In this embodiment, when viewed from theforward end of the body, the first cutting lip 12 and the second cuttinglip 13a are formed as straight edges intersecting each other at a sharppoint P, and the following features are further added:

(1) An angle α, defined as the angle between the second cutting lip 13aand the radial line N which connects the axis O of rotation and theoutermost end Q of the cutting lip 12 together as viewed from theforward end of the body 10, is set to range from 15° to 35°.

The above range is the optimal one for preventing chips from jamming andto improve chip removal efficiency. More specifically, that portion of achip cut by the second cutting lip 13a and that portion of the chip cutby the first cutting lip 12 grow at different speeds from each other, sothat as the chip is produced, it curls toward the center of the drill.If the above angle α exceeds 35°, the direction of growth of the portionof the chip cut by the first cutting lip 12 is greatly different fromthe direction of growth of the portion of the chip cut by the secondcutting lip 13a. Therefore, the chip is liable to be separated at aposition corresponding to the intersection of the first cutting lip 12with the second cutting lip 13a. Furthermore, since the angle (α+δ),defined as the angle between the second cutting lip 13a and the firstcutting lip 12, is rendered small, the cutting lips become susceptibleto chipping damage at the intersection P.

On the other hand, if the angle α is less than 15°, the ratio of thelength of the second cutting lip 13a with respect to the length of thefirst cutting lip 12, becomes large. Accordingly, the direction in whichthe chip grows is greatly affected by that portion of the chip cut bythe second cutting lip 13a, so that required differential growth rate isnot properly achieved. The longer second cutting lip 13a also causes thecutting resistance to increase.

(2) The radial rake angle δ of the cutting lip 12 at its outer end isset to range between -10° to -20° , and the ratio of a length L₁ betweenthe axis O of rotation and the intersection P to a length L₂ between theintersection P and the outermost end Q of the cutting lip 12 is set torange between 0.4:1 and 0.7:1. These ranges are the optimal ones forreducing cutting resistance to prevent the jamming of chips. Morespecifically, if the ratio L₁ /L₂ is less than 0.4, the second cuttinglip 13a cuts a narrow portion of a chip from a workpiece, so that, dueto the large force exerted on the narrow portion when extended into thespiral flute 11, the narrow portion separates from the portion of thechip cut by the cutting lip 12. On the other hand, if the ratio L₁ /L₂exceeds 0.7, the growth direction of the chip is greatly affected bythat portion of the chip cut by the second cutting lip 13a, so that asomewhat elongated chip, different from the normal transition curledfractured type, is formed. The increase of the proportion of the secondcutting lip 13a also causes the cutting resistance to increase.

Furthermore, if the radial rake angle δ exceeds -10°, then the angle(α+δ), defined as the angle between the cutting lip 12 and the secondcutting lip 13a, becomes excessively small. As a result, the portions ofchip do not interfere with each other, and the strength at the outermostend Q of the cutting lip 12 is unduly lowered. On the other hand, if theradial rake angle δ is less than -20°, the angle (α+δ), defined as theangle between the cutting lip 12 and the second cutting lip 13a,increases, and hence the chip becomes liable to divide into two piecesat the intersection 14 therebetween, and the cutting resistance isincreased.

(3) An axial rake angle θ for the second cutting lip 13a is set to befrom 0° to -5° (FIG. 14).

Since the axial rake angle θ for the second cutting lip 13a is negative,the ground surface can be left as a rake surface for the second cuttinglip 13a when the drill is resharpened, so that resharpening can beeasily effected. In addition, the strength of the second cutting lip 13acan be enhanced. However, if the axial rake angle θ is made toonegative, the cutting resistance at the second cutting lip 13aincreases. Therefore, the axial rake angle should be no more negativethan -5°.

(4) An angle λ, defined as the angle between a ground surface 15 and arake surface 16 disposed along the second cutting lip 13a, is set to be95° to 115° (FIG. 15).

Since the portion of the chip produced by the second cutting lip 13apasses into the flute 11 after it reaches the ground surface 15, itundergoes a high force at that time. If the angle λ is less than 95°,the force exerted on the portion of the chip cut by the second cuttinglip 13a is increased excessively, and hence that portion of the chiptends to separate from the portion of the chip cut by the cutting lip12. Furthermore, inasmuch as the force exerted on the chip is large, thethrust load is increased. On the other hand, if the angle λ exceeds115°, that portion of the drill body 10 adjacent to the heel is reducedtoo much, so that the chip will not be easily curled in the flute 11.

(5) A valley line 17 defined by the ground surface 15 and the rakesurface 16 disposed along the second cutting lip 13a intersects with theaxis O of the body 10 at an angle φ of 30° to 40° (FIG. 12).

If the angle φ exceeds 40°, the friction between the portion of the chipcut by the second cutting lip 13 and the ground surface 15 becomesgreat, so that the fragmentation of the chip as described above willoccur and thrust load will increase. On the other hand, if the angle φis set too small, an excess of the portion of the body 10 disposedadjacent to the heel is removed, and hence the angle should be no lessthan 30°. By setting the angle φ from 30° to 40°, separation of theportion of the chip cut by the second cutting lip 13a from the portionof the chip cut by the cutting lip 12 can be prevented, and the thrustload can be reduced substantially.

(6) An axial distance l between the outermost end Q of the cutting lip12 and the outermost end H of the heel 11b is designed to be 0.3 D to1.0 D (FIG. 12)

If the distance l is set to exceed 0.3 D, the fluid passage of cuttingoil toward the cutting part is ensured. However, if the distance lexceeds 1.0 D, that portion of the body 10 adjacent to the heel will notbe adequate.

(7) As shown in FIGS. 11 and 16, the drill body 10 includes first andsecond planar flanks 18 and 19 formed on the forward end face thereof.The first flank 18, which is provided with a relief angle β₁ of 7° to15°, is disposed along the first cutting lip 12, while the second flank19, which is provided with a relief angle β₂ greater than that of thefirst flank so as to range from 15° to 25°, is disposed along the firstflank 18. That ridge at F, defined by the intersection of the first andsecond flanks 18 and 19, is parallel to the cutting lip 12 andintersects with the axis O of the body 10.

The provision of the second flank 19 is to prevent frictional engagementbetween it and the bottom of a drilled hole, and to ensure a fluidpassage for cutting oil, so that the cutting part can be lubricated andcooled efficiently by the cutting oil. This is very advantageous for theHSS drill which is often used for heavy-duty drilling operations.

Moreover, since the first and second flanks 18 and 19 are formed to beplanar, the first and second flanks can be resharpened by surfacegrinding. This ensures a better surface finish as compared with theconical sector grinding method which is conventionally used. Inaddition, since the grinding is easily conducted, minute chipping damageat the ridge lines such as the cutting lip 12 is prevented fromoccurring during the grinding. As a result, the service life of thedrill can be extended and troubles such as early fracture can beavoided.

Furthermore, since the relief angle β₁ is set to be below 7°, flank wearcan be effectively prevented. This feature is particularly effective inthe case where a high-feed drilling operation is conducted. On the otherhand, if the relief angle β₁ exceeds 15°, the included angle ρ of thecutting lip 12 is reduced, so that the cutting lip 12 becomessusceptible to chipping or fracture.

Moreover, since the relief angle β₂ is no less than 15°, the fluidpassage for supplying cutting oil to the cutting part can besufficiently ensured, and the cooling and lubricating effect of the oilcan be improved. On the other hand, to obtain sufficient rigidity at thecutting lip, it is preferable that the relief angle β₂ should be nogreater than 25°.

In the above, the reason why the ridge F, defined as the intersection ofthe first and second flanks 18 and 19, is set to be parallel to thecutting lip 12 and to intersect with the axis O of the body is asfollows.

If the intersection ridge F is formed so that, as viewed from theforward end of the body 10, it is inclined in the direction of rotationof the body 10 with respect to the cutting lip 12, the width of thefirst flank 18 is reduced at its radially outer end portion, so that therigidity of the cutting lip is unduly reduced at its outer portion. Onthe other hand, if the intersection ridge F is formed so that it isinclined in the direction opposite to the direction of rotation of thebody 10 with respect to the cutting lip 12, the first flank 18 isenlarged in width at its outer end portion, so that a second impingementor flank-engagement tends to occur. Furthermore, if the intersectionridge F is formed so that it is inclined in the direction of rotation ofthe body 10, a chisel edge 20 with a greater chisel angle γ is formed atthe border between the two second flanks 19 as shown in FIG. 18, andhence the mechanical strength of the chisel edge 20 is lowered.Accordingly, it is preferable that the intersection ridge F should beformed so as to pass through the axis O or to be inclined in thedirection opposite to the direction of rotation of the body 10. Thefirst and second flanks can be precisely formed again upon theresharpening of the drill when the intersection ridge F is designed tointersect the axis O of the body 10.

(8) Suppose an imaginary line extends along the ridgeline of one of thesecond cutting lips 13a. Then, in the embodiment, the interveningdistance C between the imaginary line and the other second cutting lip13a is 0 to 0.3 mm, and the chisel edge 20 is formed between adjacentends of the two second cutting lips 13a. The width G of the chisel edge20 is set to be 0 to 0.4 mm (FIG. 17).

The chisel edge 20 acts so as to separate the workpiece. If the chiseledge width G is great, the thrust load increases and the cutting speedincreases at the ends of the chisel 20, so that the ends of the chiselare susceptible to chipping. Therefore, the chisel edge width G shouldbe preferably near zero, so that the above disadvantage is avoided andstable engagement can be ensured by setting it to be no greater than 0.4mm.

(9) An oil hole 22 for passage of a cutting fluid is formed through thedrill body 10 so as to extend spirally along the spiral flutes 11 (FIG.13).

With this construction, even after resharpening, the position of the oilhole 22 would remain unchanged, and hence cutting operation can bealways conducted under the same conditions. In addition, inasmuch as theoil hole 22 is spiral, the torsional rigidity of the body 10 is barelylowered. Therefore, in combination with the effects due to the optimalrange relating to the distance W, the drill can be used for heavier dutydrilling operations. These advantages could also be expected even if thedrill was provided with non-linear first and secnd cutting lips.

(10) The heel 11b, defined by the intersection of the flute 11 with theouter peripheral land 10a, is relieved to provide a chamfered faceextending along the flute 11. This chamfered face is about 0.5 mm wide.Instead of chamfering, the heel may be rounded off so as to have aradius of curvature of about 0.5 mm. With either of these machinings,the heel can be prevented from being chipped or fractured by the chipsproduced during the drilling operation.

FIGS. 19 to 21 illustrate a further modified twist drill in accordancewith a fourth embodiment of the present invention. As is the case withthe drill of the first embodiment, the drill body 10 is made of HSS orsintered metal HSS, and the web thickness T is set to be 15 to 30% ofthe drill diameter D. Those portions of flanks located rearwardly withrespect to the direction of rotation of the body 10 are subjected tocross thinning as at 13 to provide X-type ground surfaces 15, and secondcutting lips 13a each extending away from the axis O of rotation to theradially innermost end of a respective first cutting lip 12 are formedat the web portion. The distance W between the line L and the bottom 11aof the flute 11 is designed to range from 45% to 65% of the drilldiameter D, so that the second wall of the spiral flute 11 is recesseddeeply in the direction of rotation of the body 10. Furthermore, theentire surface of the drill body 10 is coated with a hard coatingcomposed of at least one material selected from the group consisting ofTiC, TiN, TiCN and Al₂ O₃. In this drill, too, inasmuch as the distanceW between the line L and the bottom 11a of the flute 11 is set to be noless than 45% of the drill diameter, the resistance exerted on the chipin the direction opposite to the direction in which the chip grows isdivided into forces to cause the chip to bend or buckle. Accordingly,the chip is prevented from being strongly compressed, so that the thrustload and cutting torque can be greatly reduced.

Furthermore, in this embodiment, the ratio of arc length A₁ of the flute11 to arc length B₁ of the land in a cross-section taken perpendicularto the axis O of the body 10, i.e., a flute-width ratio A₁ /B₁ at across-section away from the forward end, is set to range between 0.9 and1.2. That portion of the wall of the flute 11 which contacts animaginary cylinder inscribing the web portion of the body 10 isconcavely arcuately shaped at such a radius of curvature R as to satisfythe relationship: 0.15 D≦R≦0.2 D. The length of this arcuately-shapedportion, as viewed axially of the body 10, is defined by an arc with acentral angle ω from 19° to 49°, preferably from 24° to 44°, and morepreferably from 29° to 39°. Within this range of the central angle ω,not only will a chip produced by the cutting lip 12 be curledpositively, but also the friction between the chip and the wall of theflute 11 can be reduced, so that the curling of the chip can be smoothlyeffected.

As was the case with the first embodiment, the advantages of thisembodiment were verified by way of the following drilling tests.

DRILLING TEST 3

There were prepared several twist drills, having various ratios of thedistance W to the drill diameter, in accordance with the fourthembodiment of the invention. The test drill had a diameter of 12 mm anda point angle of 140°, and the radial rake angle of the cutting lip was-15°. Then the procedures, as set forth in Drilling Test 1 were repeatedunder the same conditions, and the thicknesses of chips produced duringthe drilling operation were measured. The results are shown in Table 3.

As seen from Table 3, when the distance W from the line L to the bottom11a of the flute 11 is no less than 45% of the drill diameter, thethicknesses of chips are reduced substantially as was the case with thefirst embodiment, and hence the resistance exerted on the chip can besubstantially reduced.

DRILLING TEST 4

The drill having the distance W of 53% of drill diameter and the drillhaving the distance W of 41% of drill diameter, which were prepared forDrilling Test 3, were again used, and the drilling tests were carriedout under the same conditions as those in Drilling Test 1. In this test,the thrust load, cutting torque, cutting power and maximum amplitude ofvibration of the spindle during the drilling operation were measured.The results are shown in Table 4.

It is seen from Table 4 that in the drill of the invention, the thrustload, cutting torque and cutting power are all reduced markedly incomparison with the prior art drill. Therefore, in the drill of thisinvention, since the chips can be curled without being compressedstrongly, the resistance exerted on the chips can be reduced, so thatthe cutting resistance such as thrust load can be decreasedsubstantially.

                  TABLE 3                                                         ______________________________________                                                Thickness of chips (mm)                                                          Drills of the invention                                                                       Prior art drills                                   Feed rate  Ratio (%)       Ratio (%)                                          (mm/rev.) 53       50     45      43   41                                     ______________________________________                                        0.15      0.257    0.289  0.301   0.426                                                                              0.451                                  0.25      0.349    0.370  0.391   0.523                                                                              0.546                                  0.35      0.420    0.438  0.466   0.635                                                                              0.661                                  0.45      0.511    0.523  0.550   0.761                                                                              0.790                                  0.55      0.646    0.657  0.657   0.954                                                                              0.991                                  ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                     Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      118      159    208  310    360                                 Prior art drill                                                                             220      284    350  418    485                                 Cutting torque (Kg · mm)                                             Drill of the invention                                                                       78      115    156  240    271                                 Prior art drill                                                                             121      181    219  259    351                                 Cutting power (Kw)                                                            Drill of the invention                                                                      0.42     0.70   0.85 1.23   1.45                                Prior art drill                                                                             0.71     1.23   1.41 1.59   2.05                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      3.2      2.59   2.61 2.78   2.8                                 Prior art drill                                                                             3.7      5.41   2.87 4.21   5.98                                ______________________________________                                    

Thus, in the twist drill in accordance with the above embodiment, thecutting resistance during the drilling operation can be reducedsufficiently. In addition, since the distance W between the line L andthe bottom 11a of the flute 11 is set to be no greater than 65% of thedrill diameter D, the thickness of the body 10 between the second wallof the flute disposed adjacent to the heel and the peripheral land willbe adequate, so that the drill is less susceptible to chipping damage orcracking. In addition, the torsional rigidity of the drill can bemaintained at a high level.

Moreover, since the chip is extended along the wall of the spiral flute11 and curls at the bottom 11a, the radius of curvature of the chipbecomes generally identical to the radius of curvature R of the bottomof the spiral flute 11 defined at the cross-section. If the radius ofcurvature R of the bottom exceeds 0.2 D, the chip protrudes radiallyoutward from the flute 11 and is brought into frictional engagement withthe wall of the drilled bore. As a result, the surface finish of thebore deteriorates and the chip removal efficiency is lessened. On theother hand, if the redius of curvature R is below 0.15 D, the radius ofcurvature of the chip is unduly reduced. For this reason, the bendingresistance exerted on the chip increases and is added to the cuttingresistance to increase the cutting torque and thrust load.

Additionally, since the surface of the drill body is coated with TiC,TiCN or the like, the wear resistance is sufficiently enhanced to enableheavy-duty operations to be performed.

Furthermore, since in the above drill, the cutting resistance isconsiderably reduced, the vibration of spindle of the machine tool canbe rendered small enough for the working precision to be highlyenhanced. Moreover, since the flute is so formed that the second wall isrecessed deeply in the direction of rotation of the body, thecross-sectional area is large, and hence chips can be smoothly removed.

In this embodiment, too, the first cutting lip 12 and the second cuttinglip 13a may be formed so that when viewed from the forward end of thebody 10, they appear linear and the intersection 14 is arcuately shapedat a prescribed radius of curvature r. Furthermore, the variouslimitations such as described in the third embodiment could also beadded to this embodiment.

FIGS. 22 and 23 depict a further modified twist drill in accordance witha fifth embodiment of the present invention which differs from the drillof the previous embodiments in that the drill body 10 is made ofcemented carbide, and that the body 10 has a web thickness T of 20 to35% of the drill diameter D and a flutewidth ratio A/B or 0.5 to 0.9.Additionally, the entire surface of the drill body 10 is not coated witha hard coating. Rather, only a portion of the surface of the drill body10 may be coated with the hard coating.

In the twist drill as described above, too, the distance W between theline L and the bottom 11a of the flute 11 is set to be no less than 45%of the drill diameter, and hence the resistance exerted on the chip inthe direction opposite to the direction in which the chip is extended isdivided into forces to cause the chip to bend or buckle. Accordingly,the chip is prevented from being strongly compressed, so that the thrustload and cutting torque can be greatly reduced.

These points were verified by way of the following drilling tests:

DRILLING TEST 5

There were prepared several twist drills having various ratios of thedistance W to the drill diameter. The test drill had a diameter of 12 mmand a point angle of 140°, and the radial rake angle of the cutting lipwas -15°. The Drilling Test were conducted under the followingconditions:

Cutting speed: 65 m/min.

Workpiece: Steel (JIS SCM440; Hardness:HB100 and 300 to 350).

Feed rate: 0.15, 0.25, 0.35, 0.45 and 0.55 mm/revolution.

The thickness of chips produced during the drilling operation using thedrills were measured at a point designated by S in FIG. 5. Table 5 showsthe results for the drilling of a workpiece with a hardness of 300 to350 while Table 6 shows those for the drilling of a workpiece with ahardness of 100.

As seen from Tables 5 and 6, when the distance W from the line L to thebottom 11a of the flute 11 is no less than 45% of the drill diameter,the thickness of the chips are reduced substantially. This means thatthe resistance exerted on the chip can be substantially reduced bysetting the distance W to be no less than 45% of the drill diameter.

                  TABLE 5                                                         ______________________________________                                        Workpiece of Hardness 300-350                                                         Thickness of chips (mm)                                                          Drills of the invention                                                                       Prior art drills                                   Feed rate  Ratio (%)       Ratio (%)                                          (mm/rev.) 53       50     45      43   41                                     ______________________________________                                        0.15      0.131    0.134  0.142   0.195                                                                              0.203                                  0.25      0.218    0.223  0.237   0.322                                                                              0.338                                  0.35      0.275    0.283  0.301   0.412                                                                              0.425                                  0.45      0.353    0.364  0.383   0.527                                                                              0.545                                  0.55      0.431    0.446  0.471   0.642                                                                              0.667                                  ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Workpiece of Hardness 100                                                             Thickness of chips (mm)                                                          Drills of the invention                                                                       Prior art drills                                   Feed rate  Ratio (%)       Ratio (%)                                          (mm/rev.) 53       50     45      43   41                                     ______________________________________                                        0.15      0.200    0.206  0.217   0.297                                                                              0.311                                  0.25      0.332    0.345  0.362   0.494                                                                              0.515                                  0.35      0.420    0.433  0.459   0.625                                                                              0.648                                  0.45      0.539    0.557  0.587   0.803                                                                              0.834                                  0.55      0.657    0.682  0.718   0.982                                                                              1.020                                  ______________________________________                                    

DRILLING TEST 6

The drill with the distance W of 53% of the drill diameter and the drillwith the distance W of 41% of the drill diameter, which were prepared inDrilling Test 5, were again used, and the drilling tests were conductedunder the same conditions as those in Drilling Test 5. In this test, thethrust load, cutting torque, cutting power and maximum amplitude ofvibration of the spindle during the drilling operation were measured.The results are shown in Tables 7 and 8.

It is seen from Tables 7 and 8 that in the twist drill of the invention,even though the radial rake angle is negative, the thrust load, cuttingtorque and cutting power are all reduced markedly in comparison with theprior art drill. Therefore, in the drill of this invention, since thechips can be curled without being compressed strongly, the resistanceexerted on the chips can be reduced, so that the cutting resistance suchas thrust load can be decreased substantially.

                  TABLE 7                                                         ______________________________________                                        Workpiece of Hardness 300-350                                                              Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      236      296    372  396    460                                 Prior art drill                                                                             288      332    400  480    580                                 Cutting torque (Kg · mm)                                             Drill of the invention                                                                       96      128    168  204    240                                 Prior art drill                                                                             100      136    172  224    273                                 Cutting power (Kw)                                                            Drill of the invention                                                                      1.61     2.03   2.48 2.93   3.38                                Prior art drill                                                                             1.76     2.18   2.63 3.15   3.75                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      1.98     2.22   2.16 2.88   2.52                                Prior art drill                                                                             3.66     3.30   2.58 3.24   3.48                                ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        Workpiece of Hardness 100                                                                  Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      172      216    252  312    368                                 Prior art drill                                                                             230      280    350  460    550                                 Cutting torque (Kg.mm)                                                        Drill of the invention                                                                       84      112    148  188    220                                 Prior art drill                                                                             100      130    180  230    260                                 Cutting power (Kw)                                                            Drill of the invention                                                                      1.35     0.73   2.06 2.40   2.78                                Prior art drill                                                                             1.63     2.00   2.53 3.08   3.45                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      2.64     2.82   2.76 2.52   2.70                                Prior art drill                                                                             2.9      3.0    3.6  4.3    3.9                                 ______________________________________                                    

Thus, in the twist drill in accordance with this embodiment, too, thecutting resistance during the drilling operation can be reduced. Inaddition, since the distance between the line L and the bottom 11a ofthe flute 11 is set to be no greater than 65% of the drill diameter D,the thickness of the body 10 between the second wall of the flutedisposed adjacent to the heel and the peripheral land will be thickenough, so that the drill is less susceptible to chipping damage orcracking. In addition, the torsional rigidity of the drill can bemaintained at a high level.

Furthermore, since in the above drill, the cutting resistance isconsiderably reduced, the virbration of spindle of the machine tool canbe lessened, and the working precision is thereby highly enhanced.Moreover, since the flute is so formed that the second wall is recesseddeeply in the direction of rotation of the body, the cross-sectionalarea is large, and hence chips can be easily removed.

In the above embodiment, the cutting lip 12 is formed so that it islocated forwardly from the radial line N with respect to the directionof rotation of the body 10. It, however, may be disposed rearwardly fromthe radial line N in the direction of rotation of the body 10 as shownin FIG. 24.

Furthermore, the body 10 may be made of cermet. Generally, cermet isextremely hard and is superior in wear resistance, but it is morebrittle and has less torsional rigidity in comparison to cementedcarbide (the traverse rupture strength of cermet is 130 Kg/mm² whilethat of cemented carbide is 200 Kg/mm²), and hence it has been hithertosupposed to be inappropriate for use in drills. Most specifically, if adrill is made of cermet, fatigue breakage readily develops when thedrill is subjected repeatedly to cutting torque and thrust load, so thatthe drill is broken in a very short time. However, in the drill of thisembodiment, the cutting torque and thrust load can be reduced whileenhancing the torsional rigidity, and therefore the drill can be formedof cermet. These points were verified by way of the following drillingtests.

DRILLING TEST 7

Drills having the distance W of 53% of the drill diameter and thedistance W of 41% of the drill diameter were made of cermet containingTiN, TiCN, and the like. The drilling tests were conducted under thefollowing conditions. The drill diameter was 12.5 mm, and the pointangle was 140°.

Workpiece: JIS SCM440 (Hardness: H_(RC) 30).

Cutting speed: 50 m/min.

Feed rate: 0.3 mm/revolution.

In this test, the thrust load, cutting torque, cutting power and servicelife were observed. The results are shown in Tables 9 and 10, in whichthe term "cutting length" designates the summation of the thicknesses ofall of the workpieces drilled by the test drill.

                  TABLE 9                                                         ______________________________________                                                    Drill of the invention                                                                      Prior art drill                                     ______________________________________                                        Thrust load (Kg)                                                                            312             430                                             Cutting torque (Kg · mm)                                                           166             180                                             Cutting power (Kw)                                                                          2.63            2.70                                            Service life (cutting                                                                       Still unbroken at                                                                             Broken at                                       length)       about 20 mm     about 7 m                                       ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        Feed rate (mm/rev.)                                                                         0.15     0.25   0.35 0.45   0.55                                ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      232      248    300  312    348                                 Prior art drill                                                                             290      330    385  430    480                                 Cutting torque (Kg · mm)                                             Drill of the invention                                                                      108      126    150  166    188                                 Prior art drill                                                                             115      140    155  180    220                                 Cutting power (Kw)                                                            Drill of the invention                                                                      1.97     2.18   2.44 2.63   2.93                                Prior art drill                                                                             1.98     2.26   2.48 2.70   3.08                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      3.0      2.2    2.0  2.1    2.22                                Prior art drill                                                                             3.18     3.0    2.46 2.28   3.1                                 ______________________________________                                    

As seen from Table 9 that the twist drill of the invention could drill20 m without any problems while the prior art drill was broken after itdrilled only 7 m. This is because in the drill of the invention, eventhough the radial rake angle is negative, the thrust load, cuttingtorque, and cutting power are all reduced markedly in comparison withthe prior art drill as will be seen from Table 10. Thus, the drill canbe made of cermet, and this material has the advantage of superior wearresistance.

In the above embodiment, both of the first cutting lip 12 and the secondcutting lip 13a may be formed to be linear and to intersect each otherwith an arcuately shaped corner formed therebetween. FIGS. 25 and 26illustrate such an embodiment, in which the radius of curvature r of thearcuate corner is set to range from 0.05 D to 0.15 D as is the case withthe previous embodiments.

Moreover, the various features described in the third embodiment couldas well be added to the drill of this embodiment as shown in FIGS. 27 to33. In this embodiment, however, the angle α, defined as the anglebetween the second cutting lip 13a and the line N as viewed from theforward end of the body 10, should be set to range from 20° to 40°. Theoptimal ranges for the other parameters are identical to those in thethird embodiment. Furthermore, the second cutting lip 13a and the firstcutting lip 12 may be honed as at 21 in FIG. 32. When an imaginary lineis drawn along an inner ridgeline 21a of the honing 21 of one cuttinglip, the intervening distance C between the imaginary line and an innerridge line 21a of the honing 21 of the other cutting lip is within therange of 0 to 0.3 mm, and a chisel edge 20 is formed between adjacentends 21b of the two honings 21. If the honings 21 intersect at the axis,the engagement upon the drilling is caused at two points, so that thedrill becomes susceptible to vibration and hence to chipping damage. Forthis reason, the adjacent ends of the honings 21b have to be spacedapart. The spacing C is set to be no greater than 0.3 mm in order toreduce the thrust load by reducing the chisel width G.

The adjacent ends 21b of the honings 21 may be disposed so as to almostcontact each other, but the spacing therebetween must be maintained.FIG. 33 depicts a modification of the honing, in which the adjacent endsof the two honings are made to contact each other, but their outerridgelines 21c are arcuately shaped in the vicinity of the axis O andintersect each other at the axis O. With this construction, althoughsmall, a chisel is formed at the axis O, so that the engagement with theworkpiece can be conducted at one point. The configuration of the honingmay be further modified as illustrated in FIG. 34, in which the widthdecreases as it approaches the axis. With this configuration, thechipping at the outer side of the second cutting lip 13a, where thecutting speed is great, can be prevented. Since the cutting lip 12 isstraight, the variation in the configuration of the honing 21 is small.Furthermore, in the drill of cemented carbide or cermet, too, the body10 may be coated with a hard coating of TiC, TiN, TiCN, Al₂ O₃ or thelike to enhance the wear resistance. This kind of coating also reducesfriction between the drill and the chips, and further reduces cuttingtorque and thrust load.

FIGS. 35 to 37 depict a further modified twist drill in accordance witha seventh embodiment of the invention. As is the case with the drill ofthe fifth embodiment, the drill body 10 is made of cemented carbide orcermet, and the web thickness T is set to be from 20 to 35% of the drilldiameter D. Those portions of flanks disposed rearwardly with respect tothe direction of rotation of the body 10 are ground off, as at 13, toprovide X-type ground surfaces 15, so that second cutting lips 13a, eachextending away from the axis O of rotation to the radially innermost endof a respective first cutting lip 12, are formed at the web portion. Thedistance W between the line L and the bottom 11a of the flute 11 isdesigned to range from 45% and 65% of the drill diameter D, so that thesecond wall of the spiral flute 11 is recessed deeply in the directionof rotation of the body 10. Accordingly, in this drill, too, inasmuch asthe distance between the line L and the bottom 11a of the flute 11 isset to be no less than 45% of the drill diameter, the resistance exertedon the chip in the direction opposite to the direction in which the chipgrows is divided into forces to cause the chip to bend or buckle.Accordingly, the chip is prevented from being strongly compressed, sothat the thrust load and cutting torque can be greatly reduced.Furthermore, in this embodiment, the flute-width ratio A₁ /B₁ at across-section taken perpendicular to the axis O of the body 10 is set torange from 0.9 to 1.2. Additionally, that portion of the wall of theflute 11 which contacts an imaginary cylinder inscribing the web portionof the body 10 is concavely arcuately shaped at such a radius ofcurvature R as to satsify the relationship: 0.15 D≦R≦0.2 D. The lengthof this arcuately shaped portion as viewed axially of the body 10 isdefined by an arch with a central angle ω of 26° to 56°, more preferablyof 31° to 51°, and most preferably of 36° and 46°. Within this range ofthe central angle ω, not only a chip produced by the cutting lip 12 iscurled positively, but the friction caused between the chip and the wallof the flute 11 can as well be reduced, so that the curling of the chipcan be smoothly effected.

As was the case with the first embodiment, the advantages of thisembodiment were verified by way of the following examples.

DRILLING TEST 8

There were prepared several twist drills having various ratios of thedistance W to the drill diameter. The test drill had a diameter of 12 mmand a point angle of 140°, and the radial rake angle of the cutting lipwas -15°. The drilling tests were conducted under the followingconditions:

Cutting speed: 65 m/min.

Workpiece: Steel (JIS SCM440; Hardness: H_(B) 100 and 300 to 350).

Feed rate: 0.15, 0.25, 0.35, 0.45 and 0.55 mm/revolution.

The thicknesses of chips produced during the drilling operation usingthe drills were measured at a point designated by S in FIG. 5. Table 11shows the results for the drilling of a workpiece with a hardness of 300to 350 while Table 12 shows those for the drilling of a workpiece with ahardness of 100.

                  TABLE 11                                                        ______________________________________                                        Workpiece of Hardness 300-350                                                         Thickness of chips (mm)                                                          Drills of the invention                                                                       Prior art drills                                   Feed rate  Ratio (%)       Ratio (%)                                          (mm/rev.) 53       50     45      43   41                                     ______________________________________                                        0.15      0.111    0.114  0.122   0.165                                                                              0.173                                  0.25      0.188    0.193  0.207   0.302                                                                              0.318                                  0.35      0.255    0.263  0.281   0.382                                                                              0.395                                  0.45      0.325    0.334  0.353   0.502                                                                              0.525                                  0.55      0.411    0.426  0.457   0.612                                                                              0.637                                  ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                        Workpiece of Hardness 100                                                             Thickness of chips (mm)                                                          Drills of the invention                                                                       Prior art drills                                   Feed rate  Ratio (%)       Ratio (%)                                          (mm/rev.) 53       50     45      43   41                                     ______________________________________                                        0.15      0.180    0.186  0.197   0.297                                                                              0.281                                  0.25      0.302    0.315  0.332   0.474                                                                              0.495                                  0.35      0.400    0.413  0.439   0.595                                                                              0.618                                  0.45      0.509    0.527  0.557   0.783                                                                              0.814                                  0.55      0.637    0.662  0.698   0.952                                                                              1.000                                  ______________________________________                                    

As will be seen from Tables 11 and 12, when the distance W from the lineL to the bottom 11a of the flute 11 is no less than 45% of the drilldiameter, the thicknesses of the chips are reduced substantially. Thismeans that the resistance exerted on the chip can be substantiallyreduced by setting the distance W to be no less than 45% of the drilldiameter.

DRILLING TEST 9

Drills having the distance W of 53% of the drill diameter and thedistance W of 41% of the drill diameter, which were prepared for theDrilling Test 8, were again used, and the drilling tests were conductedunder the same conditions as those in Drilling Test 8. In the test, thethrust load, cutting torque, cutting power and maximum amplitude ofvibration of the spindle during the drilling operation were measured.The results are shown in Tables 13 and 14, in which Table 13 shows theresults for the drilling of the workpiece with a hardness of 300 to 350while Table 14 shows those for the drilling of the workpiece with ahardness of 100.

                  TABLE 13                                                        ______________________________________                                        Workpiece of Hardness 300-350                                                              Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      234      297    370  393    463                                 Prior art drill                                                                             288      332    400  480    580                                 Cutting torque (Kg.mm)                                                        Drill of the invention                                                                       93      122    165  201    246                                 Prior art drill                                                                             100      136    172  224    273                                 Cutting power (Kw)                                                            Drill of the invention                                                                      1.62     2.05   2.49 2.90   3.35                                Prior art drill                                                                             1.76     2.18   2.63 3.15   3.75                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      1.95     2.33   2.09 2.85   2.55                                Prior art drill                                                                             3.66     3.30   2.58 3.24   3.48                                ______________________________________                                    

                  TABLE 14                                                        ______________________________________                                        Workpiece of Hardness 100                                                                  Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      169      212    253  315    371                                 Prior art drill                                                                             230      280    350  460    550                                 Cutting torque (Kg.mm)                                                        Drill of the invention                                                                       82      113    151  182    223                                 Prior art drill                                                                             100      130    180  230    260                                 Cutting power (Kw)                                                            Drill of the invention                                                                      1.30     1.78   2.11 2.43   2.75                                Prior art drill                                                                             1.63     2.00   2.53 3.08   3.45                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      2.61     2.78   2.72 2.50   2.68                                Prior art drill                                                                             2.9      3.0    3.6  4.3    3.9                                 ______________________________________                                    

It is seen from Tables 13 and 14 that in the twist drill of theinvention, even though the radial rake angle is negative, the thrustload, cutting torque, cutting power and amplitude of the vibration ofthe spindle are all reduced markedly in comparison with the prior artdrill. Therefore, in the drill of this invention, since the chips can becurled without being compressed strongly, the resistance exerted on thechips is reduced, so that the cutting resistance such as thrust load canbe decreased substantially.

Thus, in the twist drill in accordance with this embodiment, too, thecutting resistance caused during the drilling operation can be reducedsufficiently. In addition, since the distance between the line L and thebottom 11a of the flute 11 is set to be no greater than 65% of the drilldiameter D, the thickness of the body 10 between the second wall of theflute disposed adjacent to the heel 11b and the peripheral land 10a canbe sufficiently ensured, so that the drill is less susceptible tochipping damage or cracking. In addition, the torsional rigidity of thedrill can be maintained at a high level.

Furthermore, since in the above drill, the cutting resistance isconsiderably reduced, the vibration of the spindle of the machine toolcan be lessened, and the working precision is highly enhanced. Moreover,since the flute is so formed that that wall portion facing in thedirection of rotation of the body is recessed deeply in the direction ofrotation of the body, the cross-sectional area is large, and hence chipscan be smoothly removed.

In the above embodiment, the cutting lip 12 is formed so that it islocated forwardly from the radial line N with respect to the directionof rotation of the body 10. It, however, may be located rearwardly fromthe radial line N in the direction of rotation of the body 10 as shownin FIG. 38.

Furthermore, the body 10 may be made of cermet. The following is adrilling test similar to Drilling Test 7.

DRILLING TEST 10

The drill with the distance W of 53% of the drill diameter and the drillwith the distance W of 41% of the drill diameter were made of cermetcontaining TiN, TiCN and the like, and the drilling tests were conductedunder the following conditions. The drill diameter was 12.5 mm, and thepoint angle was 140°.

Workpiece: JIS SCM440 (Hardness: H_(RC) 30).

Cutting speed: 50 m/min.

Feed rate: 0.3 mm/revolution.

In this test, the thrust load, cutting torque, cutting power and servicelife were observed, respectively. The results are shown in Tables 15 and16.

As can be seen from Table 15, the twist drill of the invention coulddrill 20 m without any problems while the prior art drill was brokenafter it drilled only 7 m. This is because in the drill of theinvention, even though the radial rake angle is negative, the thrustload, cutting torque, cutting power and amplitude of the vibration ofthe spindle are all reduced markedly in comparison with the prior artdrill as seen in Table 15. Thus, the drill can be made of cermet, andmakes the best use of the property relating to the superior wearresistance.

                  TABLE 15                                                        ______________________________________                                                    Drill of the invention                                                                      Prior art drill                                     ______________________________________                                        Thrust load (Kg)                                                                            310             430                                             Cutting torque (Kg.mm)                                                                      162             180                                             Cutting power (Kw)                                                                          2.58            2.70                                            Service life (cutting                                                                       Still unbroken at                                                                             Broken at                                       length)       about 20 mm     about 7 m                                       ______________________________________                                    

                  TABLE 16                                                        ______________________________________                                                     Feed rate (mm/rev.)                                                          0.15   0.25   0.35   0.45   0.55                                  ______________________________________                                        Thrust load (Kg)                                                              Drill of the invention                                                                      230      245    303  314    346                                 Prior art drill                                                                             290      330    385  430    480                                 Cutting torque (Kg.mm)                                                        Drill of the invention                                                                      105      123    152  168    189                                 Prior art drill                                                                             115      140    155  180    220                                 Cutting power (Kw)                                                            Drill of the invention                                                                      1.95     2.16   2.46 2.61   2.61                                Prior art drill                                                                             1.98     2.26   2.48 2.70   3.08                                Amplitude of vibration                                                        of the spindle (μ)                                                         Drill of the invention                                                                      2.90     2.30   2.12 2.20   2.10                                Prior art drill                                                                             3.18     3.00   2.46 2.28   3.10                                ______________________________________                                    

The modifications described in the previous embodiments are alsoapplicable to this embodiment although their explanations will beomitted for the sake of brevity.

Obviously, many modifications and variations are possible in the lightof the above teachings. It is therefore to be understood that within thescope of the appended claims, the invention may be practiced otherwisethan as specifically described.

What is claimed is:
 1. A twist drill comprising:a cylindrical bodyincluding: an axis of rotation longitudinally therethrough; a forwardend which contacts a workpiece; a web thickness of 0.2 D to 0.35 D, Dbeing a diameter of said body; and a spiral flute formed in an outerperipheral surface of said body so as to extend spirally along alongitudinal length thereof to said forward end and a land disposedadjacent to said flute, and having a flute-width ratio of 0.5 to 0.9saidflute having: a first wall facing in a direction of rotation of saidbody and a second wall extending from an inner end of said first wall tothe outer periphery of said body, said first wall terminating at saidforward end in a first cutting lip having a radially outermost enddisposed on the outer periphery of said body, said second wall beingconcavely shaped when viewed from said forward end and formed so that,assuming a first line extending from said outermost end perpendicular toa second line connecting said outermost end and said axis of said body,the maximum distance between said first line and said second wall is setin a range from 0.45 D to 0.65 D.
 2. A twist drill according to claim 1,in which said body is made of a material selected from the groupconsisting of cemented carbide and cermet.
 3. A twist drill according toclaim 1, in which said body is made of a material selected from thegroup consisting of high speed steel and sintered metal high speedsteel,
 4. A twist drill according to claim 2, in which at least saidforward end of said body is coated with a hard coating.
 5. A twist drillaccording to claim 3 or claim 4, in which said hard coating is composedof at least one material selected from the group consisting of TiN, TiC,TiCN and Al₂ O₃.
 6. A twist drill according to claim 2, the wall of saidflute having an acruately shaped portion which contacts an imaginarycylinder inscribing a web portion of the drill, said arcuately shapedportion having a radius of curvature of between 0.15 D and 0.2 D.
 7. Atwist drill according to claim 3, said first wall having an arcuatelyshaped portion which contacts an imaginary cylinder inscribing a webportion of the drill, said arcuately shaped portion having a radius ofcurvature of between 0.15 D and 0.2 D.
 8. A twist drill according toclaim 1, claim 6 or claim 7, in which a web portion of the drill isground off to provide a second cutting lip extending away from the axisof said body, each of said first and second cutting lips being formed tobe linear as viewed from said forward end.
 9. A twist drill according toclaim 8, in which said first cutting lip has a radial rake angle of -10°to -20° at said outermost end.
 10. A twist drill according to claim 9,in which, when a point P is defined by the intersection of a linetangential to said first cutting lip with a line tangential to saidsecond cutting lip, the ratio of a distance between said axis of saidbody and said point P to a distance between said outermost end of saidfirst cutting lip and said point P is set to range between 0.4:1 and0.7:1.
 11. A twist drill according to claim 10, in which said forwardend includes a first planar relief surface provided with a relief angleof 7° to 15° and extending along said first cutting lip, and a secondplanar relief surface provided with a relief angle greater than that ofsaid first relief surface so as to range from 15° to 25° and extendingalong said first relief surface, the intersection of said first reliefsurface with said second relief surface being parallel to said firstcutting lip and intersecting said axis of said body.
 12. A twist drillaccording to claim 11, in which the axial rake angle for said secondcutting lip is set to be between 0° and -5°.
 13. A twist drill accordingto claim 12, in which the angle defined between a ground surface formedby the grinding and the rake surface along said second cutting lip is95° to 115°.
 14. A twist drill according to claim 13, in which the angledefined by said axis of said body and a valley line formed between saidground surface and said rake surface along said second cutting lip isset to be between 30° and 40°.
 15. A twist drill according to claim 14,in which the axial distance between said outermost end of said cuttinglip and a forward end of a heel is set to be between 0.3 D and 1.0 D.16. A twist drill according to claim 15, further comprising an oilpassageway formed in said body and extending spirally along said flute.17. A twist drill according to claim 16, in which the intersection ofthe wall of said flute and the land of said body is honed to provide ahoned face.
 18. A twist drill according to claim 17, in which said honedface is a chamfered face.
 19. A twist drill according to claim 17, inwhich said honed face is a rounded face.
 20. A twist drill according toclaim 8, in which said second cutting lip is inclined at an angle of 20°to 40° with respect to the line extending from the axis of said body tosaid outermost end of said first cutting lip.
 21. A twist drillaccording to claim 20, in which said second cutting lip is inclined atan angle of 15° to 35° with respect to the line extending from the axisof said body to said outermost end of said first cutting lip.
 22. Atwist drill according to claim 20, in which a pair of said first cuttinglips and a pair of said second cutting lips are provided and honed, andin which, assuming a first imaginary line extending from an innerridgeline of one of said honed second cutting lips and a secondimaginary line extending from an inner ridgeline of the other honedsecond cutting lip, an intervening distance between said first andsecond imaginary lines is set to range between 0.0 to 0.3 mm, said honedsecond cutting lips defining a chisel edge therebetween.
 23. A twistdrill according to claim 21, in which a pair of said first cutting lipsand a pair of said second cutting lips are provided, and in which,assuming a first imaginary line extending from an inner ridgeline of oneof said second cutting lips and a second imaginary line extending froman inner ridgeline of the other second cutting lip, an interveningdistance between said first and second imaginary lines is set to rangebetween 0.0 to 0.3 mm, said second cutting lips defining a chisel edgetherebetween.
 24. An twist drill according to claim 22 or claim 23, inwhich said chisel edge has a width of between 0.0 and 0.4 mm.
 25. Atwist drill according to claim 8, in which that corner at which saidfirst cutting lip and said second cutting lip meet forms a generallyarcuately shaped edge when viewed from said forward end.
 26. A twistdrill according to claim 25, in which the radius of curvature of saidarcuately shaped edge is between 0.05 D and 0.15 D.
 27. A twist drillaccording to claim 8, in which that corner at which said first andsecond cutting lips meet forms a sharp edge.